There are many types of silicon carbide (SiC) that have different chemical, physical, and structural characteristics that arise from the type of processing used to prepare it. Known techniques for preparing SiC articles include chemical vapor deposition (CVD), reaction bonding, sintering, hot pressing, foaming. Another more recently developed method of preparing SiC articles is pyrolysis, in which a silicon-containing polymer such as poly(methylsilane) is formed into a desired shape and then thermally decomposed in an inert atmosphere. Each processing technique may generate one or more specific crystal structures, also referred to as “polymorphs,” “polytypes,” or “phases,” e.g., cubic (zinc blende) or hexagonal crystal structures, which have different characteristics from one another.
Articles of the cubic polymorph of silicon carbide, known as beta-SiC or β-SiC, may be prepared using CVD and pyrolysis, for example. β-SiC is useful for certain applications because it has a relatively high thermal conductivity, a relatively low coefficient of thermal expansion, is relatively stable to chemicals and oxidation, and is relatively heat stable, hard, scratch resistant, electrically resistive, and resistant to radiation damage, as compared to other SiC polymorphs.
Recent concerns over nuclear fission reactor safety have led to interest in β-SiC-based materials for structural components in the nuclear reactor. Beta-SiC is an appealing nuclear material because it retains its strength at elevated temperatures, and is highly resistant to irradiation induced damage. For example, SiC fiber reinforced-SiC matrix (SiCf/SiCm) composites are an appealing structural material because they retain the desirable properties of monolithic SiC, and additionally exhibit graceful failure by pseudo-ductility associated with fiber-matrix interactions. A target application for SiC-based materials is as a fuel clad replacement for Zircaloy. A successful fuel clad must retain the fuel and fission products formed through the fuel cycle. The successful implementation of SiC-based materials in advanced reactor designs, therefore, hinges on the development of a mechanically robust joint that will adequately retain nuclear fuel within the SiC-based cladding throughout the fuel's operating cycle.
In particular, a nuclear grade joint must meet the same rigorous requirements as the SiC-based articles themselves. Furthermore, it must have well-matched thermal and irradiation induced dimensional change to the articles, as well as chemical compatibility with the fuel, fission products, and coolant. Purity and structure both play important roles in material performance in nuclear environments. High oxygen levels, and finely grained crystalline structures, in particular, lead to poor irradiation performance due to irradiation induced swelling.
Various methods have been developed to join articles of silicon carbide. For example, U.S. Patent Application No. 2008/0226868 to Pickering describes joining silicon carbide using sintered ceramics and CVD. Specifically, a ceramic is applied to the joint in the form of a paste, sol, or slurry including, e.g., 75 wt % silicon carbide powder, 7 wt % methyl cellulose as a binder, and 18 wt % water, and then sintered between 1500-2100° C. to solidify the joint. This initial joint material is α-SiC, a polytype with comparably poorer irradiation performance to β. A coating of CVD β-SiC is then optionally deposited on the joint. However, such a slurry may cause an unacceptably high amount of water (and particularly oxygen in the water) to remain in the joint, thus rendering the joint susceptible to further changes such as irradiation induced swelling, and creating a risk of contaminating nuclear fuel or other materials in the environment.
Ferraris et al., “Glass ceramic coating and joining of SiC/SiC for fusion applications,” Journal of Nuclear Materials, 258-263 (1998) pages 1546-1550, describes using a hybrid calcia-alumina (CA) glass-ceramic to join composite articles in combination with the application of either chemical vapor infiltration (CVI) or polymer infiltration pyrolysis (PIP) of SiC. However, the CA glass-ceramic has a different composition than SiC, and thus has different characteristics than SiC that may cause premature degradation of the joint over the course of the typical nuclear refueling cycle. The use of dissimilar joining materials such as glass ceramics is particularly concerning for the case of convert and burn high temperature, gas cooled fast reactors, which are designed for significantly longer refueling lifetime.
Lewinsohn et al., “SiC-based materials for joining SiC composites in fusion energy applications,” Journal of Nuclear Materials, 307-311 (2002) pages 1232-1236, describes joining silicon carbide composites by applying to the joint a mixture of hydridopolycarbosilane (HPCS, a preceramic polymer) with about 3 vol % allyl groups to promote cross-linking and 42 wt % SiC powder (F800 powder from UK Abrasives, Lot No. SZ0802A7), followed by pyrolyzing the polymer. Lewinsohn describes that the joint then was re-impregnated with polymer and pyrolyzed again. However, Lewinsohn observed cracks in the joints that would limit the strength and permit environmental degradation of the joints.
Harrison et al., “Gas-phase selected area laser deposition (SALD) joining of SiC,” Materials and Design, 20 (1999) pages 147-152, describes joining together ceramic articles with ceramic filler material using SALD, in which a high-powered laser beam is used to induce a thermal decomposition reaction of gases inside a vacuum chamber, leading to a desired solid product deposited inside the laser spot heated zone. Harrison discloses that the hermetic seals of the tested joint structures were within approximately an order of magnitude of the monolithic articles, and that the deposited material had relatively hard regions of high purity silicon carbide as well as regions of very soft deposited material. Such weak seals and soft material are clearly unsuitable for use in the harsh environment of a nuclear reactor.
As such, previously-known methods may be insufficient to prepare joints having adequate durability for use in environments that may place a great deal of thermal, mechanical, electrical, chemical, and/or radiological stress on the joint.
Accordingly, there is a need for improved joints between silicon carbide articles, particularly between articles formed of β-SiC and intended for use in environments that may place a great deal of thermal, mechanical, electrical, chemical, and/or radiological stress on the joint, such as in a nuclear reactor.